The requirement of protecting humans from the laser light or other high energy light incident to laser imaging and maintenance and service of such printers is a standard one and forms no part of this invention. Class 1 is a designation of light intensity safe to humans, and any possible exposure to a higher light intensity is broadly recognized as unacceptable. Similarly, switches which operate to provide protection when a cover is lifted or removed, are widely employed and known under the term "interlock."
U.S. Pat. No. 4,100,419 to Pedroso employs a shutter and a linkage to the shutter to protect users against exposure to laser light, but does not involve a replaceable cartridge. U.S. Pat. No. 4,135,721 to Carmerik similarly involves a shutter.